Add a flag for ex-admins to avoid banned by "admin deleted" when admin has been removed

[Sigibert]

Is your feature request related to a problem? Please describe.

From the past it has ever been like this: When u delete an admin from sourcebans their bans had the in it's description banned by "admin deleted".

Describe the solution you'd like

We should have two options:

1. Delete an admin entireley (same as before)
2. Set an admin to inactive / obsolete (whatever u will name it) so it will be referenced in old bans and we still knoe what Ex-Admin banned the player

Describe alternatives you've considered

Of course it would be possible to create an Ex-Admin group without any rights, but it feels like a workaround and not a sound solution

Additional context

none